support ed25519 keys with leap_cli

Since the release of Net::SSH in February 2017 we now have a stable implementation that supports ssh-ed25519. I can add an (extra) key for a user in provider/users/name/alice_ssh.pub, this will get deployed and works fine. A little bit of work needs to be done on lib/leap_cli/ssh/key.rb for when people use the leap user add commands.

http://net-ssh.github.io/net-ssh/:

For ed25519 public key auth support your bundle file should contain “rbnacl-libsodium“, “rbnacl“, “bcrypt_pbkdf“ dependencies.